Process for the production of elastomeric primarily isotactic polyolefins and catalysts for use in said process

ABSTRACT

A process and a catalyst for the production of elastomeric primarily isotactic polyolefins characterized by short average blocks lengths wherein olefins are polymerized in the presence of the catalyst which comprises the reaction product of a magnesium alkoxide and a tetravalent titanium halide which takes place in the presence of an electron donor, an organoaluminum compound, and a selectively control agent which is an effectively hindered heterocyclic aromatic nitrogen compound wherein the hindrance is provided by a chloro or a methoxy group.

This is a division, of application Ser. No. 484,314, filed Feb. 26,1990, now Pat. No. 5,089,573.

BACKGROUND OF THE INVENTION

This invention relates to a process for the production of elastomeric,primarily isotactic polymers and to a catalyst which can be used in suchprocess.

U.S. Pat. No. 4,335,225, issued Jun. 15, 1982, discloses a fractionableelastic polypropylene which is said to have an isotactic content of 55%or less and also to contain some sydiotactic and atactic polypropylene.This patent, and its companions on the catalyst system for making thiselastic polypropylene, contain much information about elastic-typepolypropylene compositions, although the researchers at Motecatini,especially including Giulio Natta, produced some polypropylenecompositions which exhibited some of the characteristics of elastomericcompositions. Specifically, U.S. Pat. Nos. 3,175,999, 3,257,370 and3,258,455 disclose polypropylene compositions which have someelastic-type properties.

Elastic polypropylene is different from the "normal" or more well knownpolypropylenes. These more well known types are crystalline andamorphous polypropylenes. It is normally accepted that crystallinepolypropylene generally has the isotactic and syndiotactic structure andthat amorphous polypropylene generally has considerable atacticstructure. Giulio Natta's U.S. Pat. Nos. 3,112,300 and 3,112,301describe isotactic polypropylene and give structural formulae forisotactic and syndiotactic polypropylene. The former is a straight chainof propylene units wherein the methyl groups are all aligned on one sideof the polymer chain. In the latter, the methyl groups alternate fromone side of the chain to the other. In atactic polypropylene, the methylgroups are arranged randomly on the two sides of the chain.

Almost all of the polypropylene which is used commercially iscrystalline isotactic polypropylene. These products are well known andhave been the subject of many patents and articles. Amorphouspolypropylenes, which have very little strength, are used commerciallyprimarily in adhesives and asphalt additives.

SUMMARY OF THE INVENTION

The present invention relates to a catalyst which is useful in theproduction of elastomeric, primarily isotactic polymers of olefins,especially propylene and butene. The catalyst component comprises thereaction product of a magnesium alkoxide compound, which may be of theformula MgR₁ R₂, where R₁ is an alkoxide or aryl oxide group and R₂ isan alkoxide or aryl oxide group or halogen, and a tetravalent titaniumhalide wherein the reaction takes place in the presence of an electrondonor. The catalyst is completed by an organoaluminum compound and aselectivity control agent which is an effectively hindered heterocyclicaromatic nitrogen compound wherein the hindrance is provided by a chloroor methoxy group.

DETAILED DESCRIPTION OF THE INVENTION

The "normal" well known polypropylenes discussed above are generallyhigh molecular weight materials which consist of blocks of monomer unitsof relatively or extremely long average isotactic block length (<Liso>),for example, 50 to 200 monomer units. The prior art isotactic polymers(prepared via a MgCl₂ supported catalyst) of short average isotacticblock length (about 6 to 15 monomer units) normally contain a widedistribution of polymer blocks of varying lengths and are characterizedby having relatively low tensile strength and being tacky to the touch.

The polyolefin compositions of the present invention are specificallycharacterized in that they have a narrow distribution of relativelyshort block lengths and may be characterized as being of relatively hightensile strength and non-tacky to the touch. By "block lengths" it ismeant the number of recurring monomer, in this case propylene, unitswhich, on the average, occur before there is a defect in the polymerchain. By "defect" it is meant that the symmetry of the recurring unitsis ended and there may begin a different structure (i.e. a change fromisotactic to syndiotactic) or units of another monomer may be placedtherein. It is theorized that the average block length, as determined bya numerical integration of the pentads which occur in the ¹³ C NMRspectrum, has a great effect on the properties of the polymer. Forinstance, relatively short average isotactic block lengths, i.e. about 8to 15 or even a little longer, tend to occur in a flexible and rubberypolymer which exhibits good elastic properties and is relatively strong(with tensile strengths of about 1500 to 3000 psi). On the other hand,isotactic block lengths of greater than about 50 are characteristic ofcommercial, very stiff, highly crystalline isotactic polypropylene.

U.S. Pat. No. 4,335,225, discussed above, discloses how to make anelastomeric polypropylene composition which contains up to 55%, andpreferably much less, isotactic polypropylene. This polypropylene has aninherent viscosity of 1.5 to 8, a major melting point between 135° and155° C., exhibits no yield point, has a tensile set not exceeding 150%and contains 10 to 80% by weight of a diethyl ether-soluble fractionwhich has an inherent viscosity exceeding 1.5 wherein said fraction hasan isotactic crystalline content of about 0.5% to about 5% by weight. Aspecial catalyst, which is the subject of several other related patents,is said to be required to make this material. These catalysts arehomogeneous zirconium or hafnium catalysts supported upon partiallyhydrated alumina. Such catalyst systems are difficult to work with, haveextremely low productivities (on the order of 1-2% of the productivitiesof the catalysts of this invention) and are not used commercially to anyappreciable extent.

The polyolefins of the present invention are made with a well known typeof catalyst for which there is a wealth of commercial experience andknowledge. The catalyst is comprised of the reaction product of amagnesium alkoxide compound, which may be of the formula MgR₁ R₂, whereR₁ is an alkoxy or aryl oxide group and R₂ is an alkoxide or an aryloxide group or halogen, and a tetravalent titanium halide wherein thereaction takes place in the presence of an electron donor and,optionally, a halogenated hydrocarbon. Such catalysts are well known andhave been used for several years commercially. The catalyst is completedby the addition of organoaluminium compound and a selectivity controlagent which is a hindered heterocyclic aromatic nitrogen compoundwherein the hindrance is provided by a chloro or methoxy group.

Examples of halogen containing magnesium compounds that can be used asstarting materials for the halogenating reaction are alkoxy and aryloxymagnesium halides, such as isbutoxy magnesium chloride, ethoxy magnesiumbromide, phenoxy magnesium iodide, cumyloxy magnesium bromide andnapthenoxy magnesium chloride.

Preferred magnesium compounds to be halogenated are selected frommagnesium dialkoxides and magnesium diaryloxides. In such compounds thealkoxide groups suitable have from 1 to 8 carbon atoms, and preferablyfrom 2 to 8 carbon atoms. Examples of these preferred groups ofcompounds are magnesium, di-isopropoxide, magnesium diethoxide,magnesium dibutoxide, magnesium diphenoxide, magnesium dinaphthenoxideand ethoxy magnesium isobutoxide. Magnesium alkoxides disclosed in U.S.Pat. No. 4,710,482, issued Dec. 1, 1987 to Robert C. Job, are alsopreferred for use herein. Especially preferred is Mg₄ (OCH₃)₆ (CH₃OH)_(n) X₂ where X is resorcinol or a substituted resorcinol monoanionand n is a number which preferably is 10.

Magnesium compounds comprising one alkyl group and one alkoxide oraryloxide group can be employed, as well as compounds comprising onearyl group and one alkoxide or aryloxide group. Examples of suchcompounds are phenyl magnesium phenoxide, ethyl magnesium butoxide,ethyl magnesium phenoxide and naphthyl magnesium isoamyloxide.

In the halogenation with a halide of tetravalent titanium, the magnesiumcompounds are preferably reacted to form a magnesium halide in which theatomic ratio of halogen to magnesium is at least 1.2. Better results areobtained when the halogenation proceeds more completely, i.e. yieldingmagnesium halides in which the atomic ratio of halogen to magnesium isat least 1.5. Except in cases where phenoxides (e.g. resorcinol) areused, the most preferred reactions are those leading to fullyhalogenated reaction products. Such halogenation reactions are suitablyeffected by employing a molar ratio of magnesium compound to titaniumcompound of 0.005:1 to 2:1, preferably 0.01:1 to 1:1. These halogenationreactions are conducted in the additional presence of an electron donor.An inert hydrocarbon or halohydrocarbon diluent or solvent may be usedas a partial substitute for the titanium compound.

Suitable halides of tetravalent titanium include aryloxy- or alkoxy-diand -trihalides, such as dihexanoxy-titanium dichloride,diethoxy-titanium dibromide, isopropoxy-titanium tri-iodide andphenoxy-titanium trichloride. Titanium tetrahalides are preferred. Mostpreferred is titanium tetrachloride.

Suitable halohydrocarbons are compounds such as butyl chloride, amylchloride and the following more preferred compounds. Preferred aliphatichalohydrocarbons are halogen-substituted hydrocarbons with 1 to 12,particularly less than 9, carbon atoms per molecule, comprising at leasttwo halogen atoms, such as dibromomethane, trichloromethane,1,2-dichloroethane, dichlorobutane, 1,1,1-trichloroethane,trichlorocyclohexane, dichlorofluoroethane, trichloropropane,trichlorofluorooctane, dibromodifluorodecane, hexachloroethane andtetrachloroisooctane. Carbon tetrachloride and 1,1,1-trichloroethane arepreferred aliphatic halohydrocarbons. Aromatic halohydrocarbons may alsobe employed, e.g., chlorobenzene, bromobenzene, dichlorobenzene,dichlorodibromobenzene, naphthyl chloride, chlorotoluene,dichlorotoluenes, and the like; chlorobenzene and dichlorobenzene arepreferred aromatic halohydrocarbons. Chlorobenzene is the most preferredhalohydrocarbon.

The halogenation normally proceeds under formation of a solid reactionproduct which may be isolated from the liquid reaction medium byfiltration, decantation or another suitable method. It may besubsequently washed with an inert hydrocarbon diluent, such as n-hexane,isooctane or toluene, to remove any unreacted material, includingphysically adsorbed halohydrocarbon.

The product is also contacted with a tetravalent titanium compound suchas a dialkoxy-titanium dihalide, alkoxy-titanium trihalide,phenoxy-titanium trihalide or titanium tetrahalide. The most preferredtitanium compounds are titanium tetrahalides and especially titaniumtetrachloride. This treatment increases the chloride content of thesolid catalyst component. This increase should preferably be sufficientto achieve a final chlorine atomic ratio of greater than 90% of theionic equivalents of magnesium plus titanium present in the solidcatalyst component. To this purpose the contacting with the tetravalenttitanium compound is most suitably carried out at a temperature of from60° to 136° C. during 0.1-6 hours, optionally in the presence of aninert hydrocarbon diluent. Particularly preferred contactingtemperatures are from 70° to 120° C. and the most preferred contactingperiods are in between 0.5 to 3.5 hours. The treatment may be carriedout in successive contacts of the solid with separate portions of TiCl₄.

After the treatment with tetravalent titanium compound the catalystcomponent is suitably isolated from the liquid reaction medium andwashed to removed unreacted titanium compound. The titanium content ofthe final, washed catalyst constituent is suitably between about 1.5 to3.6 percent by weight or up to about 4.5 percent. The preferred halogenatom, possibly contained in the magnesium compound to be halogenated,and contained in the titanium compound which serves as halogenatingagent and in the tetravalent titanium compound with which thehalogenated product is contacted, is chlorine.

The material used to wash the catalyst component is an inert, lighthydrocarbon liquid. Preferred light hydrocarbon liquids are aliphatic,alicyclic and aromatic hydrocarbons. Examples of such liquids includeisopentane, n-hexane, iso-octane and toluene, with iso-pentane beingmost preferred. The amount of light hydrocarbon liquid employed is 5 to100 cc/gm of procatalyst in each of 2 to 6 separate washes, preferablyabout 25 cc/gm. The resulting solid component is the procatalyst, whichis used with cocatalyst and selectivity control agent in thepolymerization process.

Suitable electron donors are ethers, esters, ketones, phenols, amines,amides, imines, nitriles, phosphines, silanes, phosphites, stibines,arsines, phosphoramides and alcoholates. Examples of suitable donors arethose referred to in U.S. Pat. No. 4,442,276. Preferred donors areesters. Preferred esters are esters of aliphatic and aromaticdicarboxylic acids, such as dimethyl carbonate, dimethyl adipate,dihexyl fumarate, dibutyl maleate, ethylisopropyl oxalate, diethylphthalate and diisobutyl phthalate. Preferred electron donors for use inpreparing the titanium constituent are the dialkylphthalates.

The primary organoaluminum compound to be employed as cocatalyst may bechosen from any of the known activators in olefin polymerizationcatalyst systems comprising a titanium halide but is most suitably freeof halogens. While aluminum trialkyl compounds, dialkylaluminum halidesand dialkylaluminum alkoxides may be used, aluminumtrialkyl compoundsare preferred, particularly those wherein each of the alkyl groups has 1to 6 carbon atoms, e.g., aluminumtrimethyl, aluminumtriethyl,aluminumtri-n-propyl, aluminumtri-isobutyl, aluminumtri-isopropyl andaluminumdibutyl-n-amyl. Alternatively, these may be used in combinationwith various alkyl aluminum halides, e.g. diethyl aluminum chloride.Preferred proportions of selectivity control agent, employed separately,in combination with, or reacted with an organoaluminum compound,calculated as mol per mol titanium compound, are in the range from 1 to100 particularly from 10 to 80.

The selectivity control agents which are necessary to achieve theadvantages of the present invention are effectively hinderedheterocyclic aromatic nitrogen compounds which are capable of serving aselectron donors and wherein the hindrance is provided by a chloro ormethoxy group. By "effectively" hindered it is meant that thesecompounds must be sterically or electronically hindered with the chloroor methoxy group to a sufficient extent so that they will produceelastomeric, primarily isotactic polypropylene which has a high degreeof elasticity even when compared to syndiotactic elastomericpolypropylenes and a higher elasticity than the so called mildlyelastomeric primarily isotactic polypropylenes.

Generally, sufficient or "effective" hindrance is provided by the chloroor methoxy constituent group or groups which are attached to the carbonatoms located on either side of the nitrogen atom in the aromatic ring.Specific examples are 2,6-dichloropyridine, 2-chloroquinoline,2-chloro-6-methoxypyridine, 2,3-dichloroquinoxaline,2,4,6-trichloropyrimidine, 2,4,5,6-tetra-chloropyrimidine,2-chlorolepidine and 6-chloro-2-picoline.

To prepare the final polymerization catalyst composition, procatalyst,cocatalyst and selectivity control agent may be simply combined, mostsuitably employing a molar ratio to produce in the final catalyst anatomic ratio of aluminum to titanium of from 1 to 150, and suitably fromabout 10 to about 150. The catalyst of this invention tends to exhibitvery good activity at much lower Al:Ti ratios, e.g., below 80:1 and evenbelow 50:1, than prior art catalysts of the same type. It may, however,be advantageous under some conditions to employ them at higher Al:Tiratios. Increasing the Al:Ti ratio tends to slightly increase catalystactivity at the expense of increased catalyst residue in the unextractedproduct. These factors will be considered in selecting the Al:Ti ratiofor any given process and desired product. In general, Al:Ti ratios inthe range of 30:1 to 100:1 and especially of about 50:1 to 80:1 will befound advantageous. SCA:Ti ratios of about 10:1 to 100:1 may be used.

Polymerization of propylene may be conducted with the catalysts of theinvention in a liquid system with an inert diluent such as a paraffinicliquid of 3 to 15 carbon atoms per molecule, or in a liquid systemcontaining propylene as sole diluent or together with a small amount ofpropane, or in vapor phase. Propylene polymerization in liquid phase isconducted at a temperature of 50° to 80° C. and at a pressure sufficientto maintain liquid conditions. In a continuous reaction system, theliquid in the reaction zone is maintained at reaction conditions,monomer is continuously charged to the reaction zone, catalystcomponents are also charged continuously or at frequent intervals to thereaction zone, and reaction mixture containing polymer is withdrawn fromthe reaction zone continuously or at frequent intervals.

In propylene polymerization, the reaction mixture is typicallymaintained at conditions at which the polymer is produced as a slurry inthe reaction mixture. The catalyst systems of this invention areextremely active and highly specific in propylene polymerization, sothat no removal of catalyst components or of atactic polymer from thepolymer product is required.

While the catalysts of this invention are particularly adapted for usein continuous polymerization systems, they may, of course, also beemployed in batch polymerization. This may be of advantage in amulti-stage polymerization in which olefin polymers and copolymers areproduced in separate reaction zones arranged in sequence.

It is well known that supported coordination procatalysts and catalystsystems of the type used herein are highly sensitive, in varyingdegrees, to catalyst poisons such as moisture, oxygen, carbon oxides,polyolefins, acetylenic compounds and sulfur compounds. It will beunderstood that in the practice of this invention, as well as in thefollowing examples, both the equipment and the reagents and diluents arecarefully dried and free of potential catalyst poisons.

The productivity of the procatalyst is determined as kg polymer/gprocatalyst in a standard one hour batch reaction; it may also beexpressed as kg polymer/g Ti. Catalyst activity is sometimes reported askg polymer/g procatalyst or Ti/hr.

The specificity towards production of syndiotactic polymer and towardsaverage block length is determined by measurements involving the pentadsobserved in the ¹³ C NMR spectrum. (See "Polymer Sequence Determination,Carbon-13 NMR Method" by James C. Randall, Academic Press, NY 1977.) Arelationship has been determined such that the average block length maybe estimated by measuring the amount of xylene soluble polymer (XS) inaccordance with regulations of the U.S. Food and Drug Administration.The XS test is carried out as follows: The sample is completelydissolved in xylene in a stirred flask by heating at 120° C. The flaskis then immersed in a water bath at 25° C. without stirring for onehour, during which the insoluble portion precipitates. The precipitateis filtered off and the solubles present in the filtrate are determinedby evaporing a 20 ml aliquot of the filtrate, drying the residue undervacuum, and weighing the residue. The xylene-solubles increases forshort block length material and may include some amorphous and lowmolecular weight crystalline material. (FDA regulations 121.2501 and121.2510, 1971). The desirable numerical value of XS for the propylenehomopolymers of this invention is typically between about 35 % and about85%.

PREPARATION OF PROCATALYSTS USED IN THIS INVENTION

Magnesium methoxide solution (12%) was prepared by dissolving magnesiummetal in methanol containing 0.125 equivalent of tetraethoxy silane(TEOS) and then filtering through a medium porosity fritted glass filterto remove the slight amount of grey suspension.

The magnesium methoxide solution (791 g, 1.10 mol) was added slowly, at60° C., to a solution of resorcinol (60.5 g, 0.55 mol) in methanol (175g) while stirring at 450 rpm with a 3 inch wide, curved teflon paddle of1.5 in² surface area. By the time 1/3 of the methoxide had been addedthe flocculant precipitate had gotten quite viscous so another 155 g ofmethanol was added. After addition was complete the reaction was stirredfor an hour then filtered. The solids were washed with methanol thenisooctane then dried under moving nitrogen to yield crystallinecylindrical rods of formula: Mg₄ (OCH₃)₆ (CH₃ OH)₁₀ (resorcinolate)₂.Partial desolvation of this precursor was carried out by boiling 40 g ofsolids in 300 g of cyclohexane, containing 120 g of tetraethoxysilane,until the volume had decreased by 20-30%.

The procatalyst was prepared by stirring 7.8 g of the partiallydesolvated precursor in 200 ml of a 50/50 (vol/vol) mixture of TiCl₄/chlorobenzene. After adding isobutylphthalate (2.5 ml, 8.7 mmol) themixture was heated in an oil bath and stirred at 115° C. for 60 minutes.The mixture was filtered hot and the solids slurried in 200 ml of freshTiCl₄ /chlorobenzene mixture. Phthaloyl chloride (0.5 ml, 3.4 mmol) andp-toluoyl chloride (0.5 ml, 3.7 mmol) were added and the mixture stirredat 115° C. After 60 minutes the mixture was filtered hot and the solidsslurried again into 200 ml of fresh TiCl₄ /chlorobenzene mixture, heatedat 115° C. for 30 min and filtered hot. The solids were then slurriedinto 100 ml of fresh TiCl₄ /chlorobenzene mixture, heated at 115° C. for10 minutes and filtered hot. The solids were allowed to cool then washedwith six 150 ml portions of isopentane and dried for 100 minutes undermoving nitrogen at 40° C. Ti=2.54%.

EXAMPLES

The cocatalyst used was triethylaluminum as a 0.28 molar solution inisooctane. 0.20 millimoles of the selectivity control agent, 0.01 mmolof the procatalyst and 0.7 mmol of the cocatalyst were mixed togetherand after 20 minutes were injected into 2700 milliliters of liquidpropylene in a reactor where the polymerizations were carried out for 90minutes at 60° C.

¹³ C Magnetic Resonance

Spectra were obtained at 135° C. on samples dissolved in1,2,4-trichlorobenzene. The spectrum reference was the mmmm methyl groupassigned at 21.68 ppm. The calculated results of % syndiotactic, %isotactic, % defective and the respective average block lengths, for theboiling isooctane fractionated polymer are shown in Tables 1 and 3.

Pentad Analysis

The pentad analyses for the examples were carried out utilizing thefollowing formulae: ##EQU1##

Tensile Properties

In order to prepare samples for tensile measurements, about 60 g of thepolymer was blended in a Brabender mixer at 190° C. with 0.3 g ofIrganox 1010 antioxidant. After cooling, a 6"×6"×2 mm plate wascompression molded at 204° C. under 5 tons pressure. Tensile bars werethen cut from the plate using a `C` die. The measurement conditions areas described in ASTM D 412-83. Roughly: Tensile set is the residualelongation imparted to a sample after stretching to an increase of 300%over its original length at a rate of 20 inches/minute and then allowingit to recover to zero load at that same rate. Tensile yield is thestress required to induce a permanent deformation in the sample. Tensileat break is the stress required to break the sample at an elongationrate of 20 inches/minute. Elongation at break is the measured elongationat the break condition.

Melt flow index was determined under condition L (2160 g, 230° C.) on aTinius Olsen Plastometer. The strands, which were formed by extrusionthrough the 3 mm die of the melt flow device, were stretched to anequilibrated length by hand pulling to nearly the break point severaltimes. The elasticity of the polymer is reported as the percentageelongation obtained upon applying a force nearly enough to break thestrand (after which, of course, the strand returns to its equilibratedlength). This measurement is essentially equivalent to the reversibleelastic elongation described by G. Natta and G. Crespi in U.S. Pat. No.3,175,999.

The analytical results of these experiments are shown in Table 1 setforth below. The table gives the percentage of isotactic andsyndiotactic content as determined by ¹³ C NMR analysis as well as thexylene soluble percentage in the polymer produced. Since each of thesepolymers contains significant amounts of both isotactic and syndiotacticblocks, I have included the average isotactic block length (L_(iso)),the average syndiotactic block length (L_(syn)) and an averagecrystalline block length (L_(ave)) which is the weighted average of theother two. In each case, the polymer contains a smaller percentage ofsyndiotactic material than isotactic material and the residual stretchvalues (a measure of elasticity) are greater than 50% and the xylenesolubles contents are greater than 60%.

                                      TABLE 1                                     __________________________________________________________________________    Cl.sup.- and .sup.- OMe Group Nitrogen Heterocycles                                            Effectively                                                                   Hindered                                                     Ex.                                                                              SCA           (Yes or No)                                                                          % Iso                                                                             <Liso>                                                                             % Syn                                                                             <Lsyn>                                                                             X.S.                                                                             Stretch                          __________________________________________________________________________    1  2-chloro-6-methoxypyridine                                                                  Y      42  11   35  7.2  62.5                                                                             --                               2  3,4,5-trichloropyridazine                                                                   N      76  27   12  6.5  20.3                                                                             14%                              3  3,4,5-trichloropyridazine                                                                   N      72  21   15  6.8  30.7                                                                             14%                              4  4,6-dichloropyrimidine                                                                      N      74  23   14  6.8  28.6                                                                              6%                              5  2,4,6-trichloropyrimidine                                                                   Y      50  11   27  6.5  59.9                                                                             70%                              6  2,4-dichloropyrimidine                                                                      N      58  17   25  7.8  46.8                                                                             26%                              7  2,4,5,6-tetrachloropyrimidine                                                               Y      62  15   18  6.3  45.4                                                                             34%                              8  2,3-dichloroquinoxaline                                                                     Y      53  14   28  7.2  48.2                                                                             28%                              9  2-chlorolepidine                                                                            Y      40  13   39  8.0  65.6                                                                             82%                              10 cyanuric chloride                                                                           Y (reactive)                                                                         74  20   12  6.6  28.3                                                                             18%                              11 6-chloro-2-picoline                                                                         Y      39  10   36  7.5  72 240%                             12 2,6-dichloropyridine                                                                        Y      47  11   28  7.0  72.7                                                                             66%                              13 2,6-dichloropyridine                                                                        Y      45  10   30  7.0  70.3                                                                             79%                              __________________________________________________________________________

All of the hindered SCA's produce polymer with relatively short blocklengths (10-15) and relatively high stretch (28-240%) while theunhindered SCA's produce polymer with longer block lengths (17-27) andrelatively low stretch (6-26%). Cyanuric chloride is thought to be tooreactive to serve as an effective electron donor. Examples 4 and 6(unhindered) and 5 and 7 (hindered) utilize very similar nitrogencompounds and yet the hindered ones produce the desired polymer whereasthe polymers produced with the unhindered ones produce polymer withhigher block lengths and lower stretch. The tensile properties of someof the polymers made according to the present invention are shown inTable 2.

                  TABLE 2                                                         ______________________________________                                                  Tbreak  Tset     Tyield                                                                              Elongation                                   Example   (psi)   (%)      (psi) at break (%)                                 ______________________________________                                         1        1998    95       752   872                                           7        2630    190      1586  743                                           9        1943    98       977   767                                          11        1674    98       723   706                                          12        2222    87       703   894                                          13        2196    93       643   873                                          ______________________________________                                    

For comparative purposes, several examples of SCA's with hinderingmethyl groups were used to polymerize propylene under similarconditions. The NMR results are shown in Table 3 and the tensileproperties in Table 4.

                                      TABLE 3                                     __________________________________________________________________________    Methyl Group Hindered Nitrogen Heterocycles                                   Example                                                                            SCA        % Iso                                                                             <Liso>                                                                             % Syn                                                                             <Lsyn>                                                                             X.S.                                                                             Stretch                                  __________________________________________________________________________    A    2,3-dimethyquinoxaline                                                                   34  13   45  8.0  61.4                                                                             58%                                      B    2,4-dimethyquinoline                                                                     37  13   45  8.4  61.3                                                                             78%                                      C    2,6-lutidine                                                                             25  10   54.5                                                                              8.7  67.6                                                                             68%                                      __________________________________________________________________________

All produce primarily syndiotactic polymer whereas the SCA's of thepresent invention produce primarily isotactic polymer. The SCA's ofexamples A, B and C differ from those of 8, 9 and 12 (and 13) only inthat a methyl group has replaced the hindering chloro group and yet onegroup leads to primarily syndiotactic and the other to primarilyisotactic polymer.

                  TABLE 4                                                         ______________________________________                                                  Tbreak  Tset     Tyield                                                                              Elongation                                   Example   (psi)   (%)      (psi) at break (%)                                 ______________________________________                                        A         2089    99       914   876                                          B         2385    121      1049  846                                          C         1554    41       730   750                                          ______________________________________                                    

1.8 liters of 1-butene was polymerized with 2 different SCA's at thesame conditions as above. The results shown in Table 5 show that theCl³¹ hindered SCA (Ex. 14) produces primary isotactic polybutylene whilethe methyl hindered SCA (Ex. D) produces primarily syndiotacticpolybutylene.

                  TABLE 5                                                         ______________________________________                                        Exam-               %                                                         ple   SCA           Iso    <Liso> % Syn <Lysn>                                ______________________________________                                        14    2,6-dichloropyridine                                                                        56%    13     18%   6.1                                   D     2,6-lutidine  27%    10     38%   6.9                                   ______________________________________                                    

What is claimed is:
 1. A catalyst for use in the polymerization toelastomeric primarily isotactic polyolefins characterized by shortaverage block lengths which comprises:(a) The reaction product of amagnesium alkoxide and a tetravalent titanium halide wherein thereaction takes place in the presence of an electron donor, (b) Anorganoaluminum compound, and (c) A selectivity control agent which isselected from the group consisting of 2,6-dichloropyridine,2-chloroquinoline, 2-chloro-6-methoxy pyridine, 2,3-dichloroquinoxaline,2,4,6-trichloropyrimidine, 2,4,5,6-tetrachloropyrimidine,2-chlorolepidine and 6-chloro-2-picoline.
 2. The catalyst of claim 1wherein the tetravalent titanium halide is titanium tetrachloride. 3.The catalyst of claim 2 wherein the magnesium alkoxide is a compound ofthe formula MgR₁ R₂, where R₁ is an alkoxide or aryl oxide group and R₂is an alkoxide or aryloxide or halogen.
 4. The catalyst of claim 3wherein both R₁ and our R₂ are ethoxide.
 5. The catalyst of claim 2wherein the magnesium alkoxide is a magnesium compound of the formula[Mg₄ (OR₃)₆ (R₄ OH)₁₀ ]X where X is a couterion or ions having a totalcharge of -2 and R₃ and R₄, which may be the same or different, areselected from alkyl groups of 1 to 4 carbon atoms.
 6. The catalyst ofclaim 5 wherein R₃ and R₄ are methyl groups and X is resorcinol or asubstituted resorcinol monoanion.